What is a .vpax file?

A .vpax file is a VertiPaq Analyzer export. VertiPaq is the column store engine inside Power BI and Analysis Services, and the export describes how a model is stored in it: table by table, column by column, in bytes.

Who makes it

DAX Studio, with Advanced → Export Metrics. Bravo for Power BI. Tabular Editor 3. All three write the same format.

What is inside

A zip of JSON files.

• DaxVpaView.json: for every table and column, the row count, the number of distinct values, the size split into data, dictionary and hierarchy, the encoding, and the relationships with their cost.
• DaxModel.json: the model as VertiPaq Analyzer sees it: tables, columns, measures, relationships.
• Model.bim: a copy of the model's definition.

What is not inside

Data values. A column's dictionary size says how many distinct values it has, not what they are.

What people use it for

Finding the columns that take the most memory. Spotting high-cardinality keys and columns nobody uses. Seeing which relationships are expensive. The SQLBI tools built the format for this kind of work.

What Lantern does with it

The size of each table sets the size of its dot in the portrait, so the weight of a model is visible at a glance: the fact table is the big dot, the lookup tables are the small ones.

What to be careful with

Names, sizes and, through the copy of the definition, your DAX and M. Lantern reads the file inside your browser and sends nothing.